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¹ Istanbul University Faculty of Fisheries, Department of Aquaculture, Ordu Cad., No: 200, 34470, Laleli-Istanbul, Turkey Viewed : 4326 - Downloaded : 3916 In this study, the effects of dietary composition and seasonal variation on fillet composition and some morphological indices of wild and cultured sea bass (average weight, 361 g) were investigated. Experimental fish were fed with two commercial pelleted feeds and two commercial extruded feeds in summer, winter and spring seasons of year 2004 at four marine fish farms in Aegean region in Turkey. No significant differences were found among the proximate composition of feed samples seasonally. The crude protein content (about 45%) in the commercial feeds was found similar (P>0.05), whereas the crude fat content (about 20%) in the extruded feeds was significantly higher (P<0.05) than those (about 13%) in pelleted feeds. No significant differences (P>0.05) were found in the condition factors (CF) of the cultured and wild sea bass, neither among different feeds nor among different seasons. The viscerosomatic index (VSI) and hepatosomatic index (HSI) values or visceral and liver lipid content of fish fed with the extruded feeds were higher (P<0.05) than in those fed with the pelleted feeds and wild fish. The VSI and HSI values of wild fish were similar (P>0.05) to the values of fish fed with lower fat diet. The visceral lipid content of cultured and wild fish significantly increased in summer, while the liver lipid content significantly increased in winter (P<0.05). No significant differences (P>0.05) were found in the ash content of the cultured and wild sea bass, neither among different feeds nor among different seasons. Crude protein levels of the fish fillet did not differ (P>0.05) among the fish fed with different commercial feeds or during different seasons, although the crude protein levels in the cultured fish were significantly higher (P<0.05) than that of the wild fish. A high positive correlation (r = 0.84, P<0.01) was found between the fillet lipid levels and the dietary lipid content. Furthermore, the lipid content of cultured fish fillet was not affected (P>0.05) by different seasons whereas the fillet lipid content of the wild fish during summer was slightly higher (P<0.05). Finally, the results of chemical analysis showed that the fillet composition of cultured and wild sea bass were good sources of protein and lipid in each of the three seasons. Keywords : Cultured and wild sea bass, Dicentrarchus labrax, dietary effect, seasonal variation, fillet quality